XIII 



PHYLUM CHORDATA 



85 



converting starch into sugar. There are also two large and highly 

 characteristic digestive glands in the abdominal cavity, both 

 developed as outpushings of the intestine, but differing greatly 

 from one another in their fully developed state, both in outward 

 appearance and in histological structure : these are the liver and 

 the pancreas. 



The liver (Fig. 769, A, Ir.) is a dark-red organ of relatively 

 immense size : it not only secretes a digestive juice, the bile, which 

 has the function of emulsifying fats, but also forms an amyloid 

 substance called glycogen or animal starch, which, after being stored 

 up in the liver-cells, is restored to the blood in the form of sugar. 

 The liver is formed of a mass of polyhedral cells (Fig. 779, I.) with 

 minute intercellular spaces which receive the bile secreted from 

 the cells and from which it passes to the ducts (b). The pancreas 

 (Fig. 769, A, pn.) is a racemose gland, and secretes pancreatic juice 

 which acts upon pro- j^ 



teids, starch, and c \ / 



fats. The ducts of 

 both glands usually 

 open into the anterior 

 end of the intestine : 

 that of the liver (b. d.) 

 generally gives off a 

 blind offshoot ending 

 in a capacious dilata- 

 tion, the gall-bladder 

 (g. b.) in which the 

 bile is stored. We 

 thus have one or 

 more hepatic ducts 

 conveying the bile 

 from the liver and 

 meeting with a cystic duct from the gall-bladder, while from the 

 junction a common bile-duct leads into the intestine. 



Another important and characteristic organ in the abdomen of 

 Craniata is the spleen (spl.), a gland-like organ of variable size 

 and shape, attached to the stomach by a fold of peritoneum, but 

 having no duct. It is formed of a pulpy substance containing 

 numerous red blood-corpuscles, many of them in process of dis- 

 integration : dispersed through the pulp are masses of leucocytes 

 which multiply and pass into the veins. 



Two other ductless glands are formed in connection with the 

 enteric canal. The thyroid (thd.) is developed as an outpushing 

 of the floor of the pharynx which becomes shut off, and forms, in 

 the adult, a gland-like organ of considerable size. Its final posi- 

 tion varies considerably in the different classes. It has been com- 

 pared with the endostyle of Tunicata and of Amphioxus, which, as 



VOL. 11 G 



a. 77 9. Diagram of structure of liver, b. a small branch of 

 hepatic duct ; b'. its ultimate termination in the intercellular 

 spaces ; c. blood-capillaries ; I. liver-cells. (From Huxley's 

 Physiology.) 



